What are the symptoms of someone about to go into coma?

Lethargy, drowsiness. There are stages of coma, measured on the glasgow coma scale. So symptoms will depend on the depth of the coma. However as one goes into a coma it may be sudden loss of consciousness due to acute head trauma, or a gradual slide as with a subdural bleed. The latter is more like a person getting progressively drunk, losing coordination or slowly falling asleep.